1. Field of the Invention
The present invention relates to a process and to a circuit arrangement for producing conference connections between, respectively, three conference parties in a PCM time multiplex switching system, in which sum code words are fed to the conference parties, respectively, which sum words are formed from PCM words supplied by the respective other conference parties, possibly after conversion of the PCM words into linear code representations and reconversion into non-linear code representations subsequent to the summation.
2. Description of the Prior Art
The formation of sum code words in which, respectively, the specific portion of the conference party to whom the sum code word is fed is lacking, is particularly necessary in those cases in which loud room noises occur at the individual parties, which noises make themselves noticeable as intereference beyond the side-tone level which is desirable per se.
In a known process of this type, as disclosed in U.S. Pat. No. 3,984,643, sum code words are first formed which contain the sum portions of all conference parties. Subsequently, in the respectively same pulse frame of the time multiplex operation, several remainder sums are formed by subtraction of individual conference parties' respective portions, which sums are sent to, respectively, that conference party whose own portion is not contained in this remainder sum. The subtraction of the respective portions is accomplished with the aid of the same device with which the addition was carried out, for which purpose the operational assigned bit of the respective PCM word representing the portion of that party is converted and then an addition of the formed PCM word is undertaken.
In contrast to the double use of an arithmetic unit for addition and subtraction, which is possible with this process, is a relatively large time expenditure, with the consequence that the adder must be able to operate very rapidly so that the operations described can be performed within a pulse frame, which, in turn leads to a relatively high price for such devices.
In smaller auxiliary exchanges it is frequently sufficient if the possibility of being able to carry on conference discussions is limited to three conference parties.